Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933573Ab3JOUVx (ORCPT ); Tue, 15 Oct 2013 16:21:53 -0400 Received: from mail-wg0-f43.google.com ([74.125.82.43]:47330 "EHLO mail-wg0-f43.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759600Ab3JOUVk (ORCPT ); Tue, 15 Oct 2013 16:21:40 -0400 Date: Tue, 15 Oct 2013 22:21:36 +0200 From: Frederic Weisbecker To: Andrew Morton Cc: Steven Rostedt , LKML , Linus Torvalds , "H. Peter Anvin" , Ingo Molnar , Peter Zijlstra , Thomas Gleixner , "Liu, Chuansheng" Subject: Re: [PATCH] bug: Use xchg() to update WARN_ON_ONCE() static variable Message-ID: <20131015202134.GB3269@localhost.localdomain> References: <20131015155806.04e2613f@gandalf.local.home> <20131015131259.3051743890ce9b1b9dae9763@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20131015131259.3051743890ce9b1b9dae9763@linux-foundation.org> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2135 Lines: 52 On Tue, Oct 15, 2013 at 01:12:59PM -0700, Andrew Morton wrote: > On Tue, 15 Oct 2013 15:58:06 -0400 Steven Rostedt wrote: > > > The WARN_ON_ONCE() code is to trigger a waring only once when some > > condition happens. But due to the way it is written it is racy. > > > > if (unlikely(condition)) { > > if (WARN(!__warned)) > > __warned = true; > > } > > > > The problem is that multiple CPUs could hit the same warning and > > produce multiple output dumps of the same warning, or an interrupt could > > happen and hit the same warning and do the warning in the middle of a > > previous one, especially since the WARN() does a dump of the current > > stack. > > > > Even more of a problem, a recent WARN_ON_ONCE() that was in the page > > fault handler triggered and the stack dump of the WARN() caused the > > same WARN_ON_ONCE() get hit again. Since the __warned = true is not > > updated until after the WARN() is completed, each WARN() triggered > > another page fault causing the stack to be filled and crashed the box. > > > > The point of WARN_ON() is to warn the user and not to crash the box. > > > > The easy fix is to update the __warned variable with a xchg(). This way > > only one WARN_ON_ONCE() will actually happen, and prevents any issues > > of the WARN() causing the same WARN() to be hit and crash the system. > > printk_once() has the same issue, and probably other places. > > Is there some sneaky way of doing this operation as a common thing, > rather than open-coding it everywhere? Something like > > #define ONCE() ({ > static int state; > int ret; > > ret = !xchg(&state, 1); > ret; > }) Oh, I have a pending patchset that I worked on a few weeks ago which does that. I did not post it because it made WARN_ONCE using the unlikely text section, but the diffstat was nice. I'm going to post that as RFC just in case. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/